The braking device generally used e.g. in an elevator hoisting machine is a machine brake which mechanically engages a rotating part of the hoisting machine. The machine brake may be constructed e.g. as a drum brake or a disc brake. The braking function of the machine brake is activated when the supply of power to the electromagnet of the brake is interrupted e.g. by means of a relay or contactor. The relay/contactor which interrupts the supply of power to the electromagnet of the brake is usually of a type that only remains in the conducting state for as long as power is being supplied to the control terminal of the relay/contactor. Supply of power to the control terminal takes place via the elevator safety circuit and is interrupted when the safety circuit is opened. When the brake is activated, a brake shoe is pressed mechanically against a braking surface to brake the motion of a rotating part of the hoisting machine.
As the machine brake of a hoisting machine usually also functions as a safety device consistent with the requirements of safety regulations, the aim is to design the machine brake/brakes in such a way that the operation of the machine brake/brakes will not cause any danger e.g. due to a failure situation or some other functional irregularity. For example, the braking apparatus of the hoisting machine may comprise two separate brake shoes and in conjunction with these two separate sets of push springs to ensure that the braking power will not be completely lost if one of the brake shoes/push springs fails.
The safety regulations relating to the hoisting function of an elevator have changed because new implementations have become feasible due to the development of technology and, on the other hand, e.g. also because elevators are increasingly being constructed without a machine room. Experts working under the applicant's control are endeavoring to develop the braking apparatuses of hoisting machines by analyzing possible failure situations and improving the functioning of the braking apparatus in different operational situations of the elevator. Some of the issues for further development detected as regards the operating safety of the braking apparatus of a hoisting machine are the following:                It is important to ensure that the braking power is sufficient to stop the movement of the elevator car in all operational situations, even in a situation where the elevator car is carrying an approx. 25-percent overload while running in the down direction. The problem is that the braking power may gradually diminish e.g. due to dirt, grease or the like having got on a brake shoe or a braking surface of the hoisting machine.        Especially in elevator systems without machine room, but also in systems with machine room, care has to be taken to ensure that a brake failure—e.g. a fault in an electromagnet, push spring or brake shoe of the brake—will not result in a risk to a serviceman working in the elevator shaft pit or on the top of the elevator car.        If the brake functions correctly, the braking power will suffice to stop a downwards moving elevator car carrying a 25-percent overload e.g. in an emergency stopping situation. This leads to the consequence that an upwards moving elevator car with full load may stop in an emergency stopping situation with a deceleration rate even dangerously high for the passenger. This deceleration problem is particularly notable in elevator systems without counterweight.        Reliability of the brake control is equally important for safe operation of the brake as reliability of the operation of the mechanical brake components. A control failure that prevents activation of the brake/brakes is particularly dangerous. Therefore, any failure of the braking apparatus should always take place in a so-called safe failure direction, so that a failure situation always leads to activation of the brake/brakes. It is also necessary to make sure that a short circuit or a similar cross connection in the brake control circuit will not prevent activation of the brake/brakes.        